Electric circuit protection devices, such as circuit breakers, for example, may be used to interrupt current flow relatively quickly to a circuit that is being protected by the protection device upon the occurrence of an overload, such as a fault. This has been referred to in the industry as tripping the breaker. Once the condition that gave rise to the trip has been corrected, the circuit breaker may be reset, such as by appropriately manipulating an operating handle in the case of a manually resettable circuit breaker.
When a circuit breaker trips because of a dead short fault, it is believed that relatively large magnitudes of electric current may flow through the interruptable current path of a circuit breaker, so as to initiate the trip. However, the circuit breaker integrity must be maintained until it finally completes the trip. Moreover, because it is resettable, a circuit breaker must be capable of maintaining its integrity over its specified life, during which the circuit breaker may be subjected to multiple instances of tripping and resetting.
The internal construction of a circuit breaker comprises various individual electric parts. Some of these parts are joined together by welding or brazing. A known method for joining certain parts comprises high temperature welding. An example of high temperature welding is resistance welding wherein pressure is applied to the parts at a location where they are to be joined, and welding current is passed through the location to create temperatures sufficiently high to cause a certain degree of localized material melting and flow migration between the parts so that upon termination of the welding current, the molten mass solidifies to create the joint. It is believed that the thermal effects of resistance welding may act on the parts in a manner that undesirably affects one or more physical properties of at least one of the parts being joined. One example of such a side effect comprises some annealing of all or a portion of a part.
Another known method for joining certain parts comprises ultrasonic welding wherein pressure is applied to the parts at a location where they are to be joined, such as by clamping them in a suitable fixture in an ultrasonic welder. Ultrasonic energy is then applied to that location to create a certain upsetting and flow of material between the parts which ceases upon termination of the application of the ultrasonic energy, thereby creating the joint. It is believed that one advantage of ultrasonic welding is the elimination or at least attenuation of annealing of the parts being joined.
In any particular application, it is believed that the choice of using either resistance welding or ultrasonic welding depends on the composition of the parts being joined. For example, in a circuit breaker application where an electric contact is to be joined to another electric part, such as a terminal or a movable contact arm or blade, if the contact comprises the combination of a refractory element, such as tungsten or molybdenum, and an electric conductor, such as silver or copper, and the other part comprises, either predominantly or exclusively, a non-ferrous electric conductor, such as copper, the disposition of an attachment agent on a face of the contact that is to be placed in intimate surface-to-surface contact with the non-ferrous conductor is generally believed to be suitable for high-temperature welding. The attachment agent should be compatible with the contact's conductor material; for example, being fine silver when the contact conductor comprises silver. It is believed, however, that such materials are inappropriate or at least not optimally suited for ultrasonic welding. It is also believed that refractory-based materials are at least generally not optimal candidates for the alloying that is necessary to create an acceptable joint by ultrasonic welding.